Aneurysms are a common defect in the vascular system that account for a wide range of symptoms. When they occur in the brain, aneurysms can cause stroke or death, as well as several well-known neurological defects such as loss of sight, hearing or balance. The treatment of aneurysms within the brain may be accomplished with a number of therapies currently available. Open surgical techniques require cutting into the skull and lifting brain matter away from the aneurysm so that the aneurysm may be accessed, clipped or sutured closed and cut away. These techniques are very risky, and are reserved until absolutely necessary because of high mortality and high chance of neurological defects caused by the operation itself.
The high risk and generally unsatisfactory results of open surgery on aneurysms (especially within the brain) have led researchers to develop techniques for treating aneurysms from inside the blood vessels. Endovascular and percutaneous insertion of devices avoids the danger of open surgery on the brain, but presents technical challenges. Grafts, stents and combinations of stents and grafts have been proposed for use in larger vessels such as the aorta and the peripheral arteries. The purpose of these devices is to close off the aneurysm from the circulatory system to prevent rupture and promote resorption of the mass of the aneurysm. These devices tend to be bulky and generally unsuitable for the small environment of the brain. Our co-pending U.S. patent application Ser. No. 08/707,996, filed Sep. 18, 1996 and U.S. Patent application No. 08/762110, filed Dec. 9, 1996 present several stent designs and stent delivery systems particularly suited to be used within the brain.
Another approach to treating aneurysms, suitable for treatment within the brain, is stuffing the aneurysm with foreign material to promote thrombus within the aneurysm and eventually eliminate the threat of ruptures and promote resorption of the aneurysm sac. As early as 1975, metal coils were successfully used to occlude the renal arteries. Gianturco, et al., Mechanical Devices for Arterial Occlusions, 124 Am. J. Roent. 428 (1975). The purpose of the coil is to encourage quick formation of a thrombus (a blood clot) around the coil. The coils are currently in use for a wide range of treatments, and are referred to variously as occlusive coils, embolization coils, or Gianturco coils. Embolization coils of appropriate size for placement within aneurysms are commercially available from Target Therapeutics, Inc. and Cook, Inc. Embolization coils made with electrolytic mechanisms for detachment from the delivery catheter are referred to as GDC's or Guglielmi Detachable Coils. The use of GDC's is illustrated, for example, in Klein, et al., Extracranial Aneurysms and Arteriovenous Fistula: Embolization with the Guglielmi Detachable Coil, 201 Radiology 489 (1996). Use of the GDC coils within the brain is illustrated, for example, in Casasco, et al., Selective Endovascular Treatment Of 71 Intracranial Aneurysms With Platinum Coils, 79 J. Neurosurgery 3 (1993).
Because Gianturco and Guglielmi coils are often used to occlude aneurysms in critical areas of the body, it is important that they remain in place where they are implanted. However, migration of the coils after placement is a common but dangerous problem encountered with these coils. Watanabe, Retrieval Of A Migrated Detachable Coil, 35 Neuro. Med. Clin. 247 (1995) reports the migration of a coil into the basilar artery from a placement in the superior cerebellar artery. Halbach, et al., Transarterial Platinum Coil Embolization Of Carotid Cavernous Fistulas, 12 AJNR 429 (1991) reports the migration of a coil from the internal carotid artery. Migration is particularly common with coils placed in wide neck aneurysms. The possible migration of coils is a danger that must be considered in every procedure, and actual migration can be life threatening complication, since embolization at an unwanted site could occlude a critical blood flow. Migration of the coil may also represent a failure of the intended therapeutic procedure.
Our co-pending U.S. patent application Ser. No. 08/813,614, filed Mar. 7, 1997, discloses a hoop stent for holding open blood vessels subject to occlusive disease. The stent, which is made of a single wire, and the delivery mechanism for the stent allow the stent to placed with a low profile by stretching the stent along its long axis rather than compressing it radially. A wide variety of other stent designs have been proposed for use in the vascular system. Typically, the stents are used to hold open a length of blood vessel which has been closed or occluded by some growth within the blood vessel. Balloon expandable stents and self-expanding stents are commercially available and have been used successfully for treatment of a number vascular diseases. Das, Stent, U.S. Pat. No. 5,554,181 (Sep. 10, 1996) shows a wire stent having a number of hoops all attached to a radially disposed spine, all of which may be formed of a single wire. Likewise, Hillstead, Endovascular Stent Apparatus and Method, U.S. Pat. No. 4,856,516 (Aug. 15, 1989). The stents are folded upon a catheter pusher and retained within a catheter sheath before release into the body. These stents must be radially compressed to fit within the catheter sheath, and expand elastically or may be expanded inelastically by a balloon. They are not susceptible to being stretched or elongated in along their long axes to reduce their overall diameter.